1. Field of the Invention
This invention relates to pulse compression processors used for receivers of radars, and more specifically, to techniques for realizing pulse compression of the received signals with sidelobe-free as well as minimum S/N loss.
2. Description of the Related Art
A radar signal processor adopting a pulse compression method used for radars is shown in Japanese Patent Application Laid-Open H04-357485. This radar signal processor which transmits chirp signals (linear FM modulated signals) as transmission signals to relatively moving targets detects the targets from the Doppler frequency components extracted from the signals reflected by the targets.
This pulse compression method is used for converting chirp signals received by a receiver into short pulse signals by pulse compression filters matching these chirp signals. Therefore, since the method has some advantages such as longer distance detection, higher range resolution, and interference signal suppression, it is applied to many radar systems.
Such a conventional radar, called chirp radar, generally has performances evaluated by the shape of the compressed pulse signal, in particular, the width of a main-lobe (main-lobe width) and the level of sidelobe (sidelobe level), and the amount of S/N loss in the peak value of main-lobe.
As is well known, in general, main-lobe width is preferable to be narrow because of the increase of radar resolution. Further, a sidelobe level that indicates pseudo targets such as ghost echoes and clutters is preferable to be low. Of course, although S/N loss that deteriorates radar sensitivity is clearly preferable to small, the increase of the transmitted power in order to keep radar sensitivity is generally very expensive.
In the process of pulse compression, the signals received by a receiver are usually weighted by window functions to suppress the sidelobe level of the received signals (see Section 4.6.3 in the text entitled “Radar Handbook”, 2nd Edition, written by M. I. Skolnik, published by McGraw-Hill, Inc. (1990)). Typical window functions have properties shown in Table 10.8 in the text quoted above. However, the suppression of sidelobe level increases S/N loss because main-lobe width becomes broadened. This leads us to the finding of the complementary relationship between main-lobe width and sidelobe level. Thus, when designing radar systems, we are forced on the trade-off between them.
There are other methods for suppressing sidelobe level as follows: first, constructing inverse filters that minimize mean squared errors from the expectation shape of the correlation output of the compressed pulse signals with desired sidelobe level; secondly, subordinately connecting sidelobe eliminating filters to pulse compression filters, and thirdly, subtracting correlation output with one sample shifts from the correlation output of the compressed pulse signals. However, since these methods aim at only sidelobe suppression, they do not assure the minimization of S/N loss because they consider the S/N loss calculated after the filter design finished.
Additionally, the following techniques are already known: first, in binary phase code modulating method, minimizing S/N loss with allowable maximum peak sidelobe level; and secondly, in the same method, minimizing peak sidelobe level with desired S/N loss. In these techniques, the method of steepest descent is used for obtaining an optimum solution of S/N loss or peak sidelobe level. However, it is hard to converge them with optimum solutions because computational efforts increase with the increase of the length of input code sequences, which are complex numbers, in the method of steepest descent.